In the case of catalysts containing ruthenium or ruthenium compounds, the reaction with sulfur compounds leads in many typical applications to an irreversible reduction of the activity, which is attributable in conventional opinion to poisoning.
A typical field of use for a catalyst containing ruthenium or ruthenium compounds is the preparation of chlorine by gas-phase oxidation of hydrogen chloride with oxygen:4HCl+O22Cl2+2H2OThis reaction is an equilibrium reaction. The position of the equilibrium shifts to the detriment of the desired end product with increasing temperature. It is therefore advantageous to use catalysts which have the highest possible activity and enable the reaction to take place at a lower temperature.
The first catalysts for the hydrogen chloride oxidation contained copper chloride or copper oxide as an active component and were described by Deacon as long ago as 1868. However, these had only low activities at low temperature (<400° C.). Although it was possible to increase the activity thereof by increasing the reaction temperature, a disadvantage was that the volatility of the active components led to rapid deactivation.
Since it was not possible to make substantial progress up to the 1960s in spite of immense research activities in this area, the Deacon process named after the discoverer was eclipsed by the chloralkali electrolysis. Up to the 90s, virtually the total production of chlorine was effected by electrolysis of aqueous sodium chloride solutions [Ullmann's Encyclopedia of Industrial Chemistry, seventh release, 2006]. However, since the worldwide demand for chlorine is currently growing more strongly than the demand for sodium hydroxide, the attractiveness of the Deacon process is unbroken since hydrogen chloride can be reused thereby for the preparation of chlorine, which results in large amounts thereof as a coproduct, for example in the phosgenation of amines.
Substantial progress in the area of hydrogen chloride oxidation was made by the discovery of ruthenium or ruthenium compounds as catalytically active components, which were described for the first time in 1965 in DE 1567788. Particularly in the provision of a suitable support, considerable progress has since been made. Titanium dioxide, the use of which is described, for example, in the application EP 743277 A1, and tin dioxide, the use of which is evident, for example, from the application DE 10 2006 024 543 A1, appear particularly suitable as supports.
Further typical fields of use for catalysts containing ruthenium or ruthenium compounds are the (selective) oxidation of carbon monoxide and the purification of exit air. U.S. Pat. No. 7,247,592 B2 describes a catalyst containing ruthenium or ruthenium compounds for the selective oxidation of carbon monoxide. U.S. Pat. No. 7,318,915 B2 discloses the use of catalysts containing ruthenium or ruthenium compounds for combined use in the area of exit air treatment. There, the catalyst described oxidizes carbon monoxide and volatile hydrocarbons, while nitrous gases are also reduced.
A multiplicity of further uses for catalysts containing ruthenium or ruthenium compounds is also known. Particularly in the synthesis of key organic chemicals from mineral oil fractions, natural gas or coal, catalysts containing ruthenium or ruthenium compounds often play a key role.
A major problem with the use of catalysts containing ruthenium or ruthenium compounds is evidently the sensitivity thereof to poisoning with sulfur. A sulfur load in the entry stream may be characterized, for example, by sulfur-containing raw materials (e.g. mineral oil fractions, natural gas, coal) or upstream processes (e.g. gas drying with sulfuric acid, sulfur-containing active carbon). WO 2007 066 810 A1 discloses, for example, that it is of decisive importance for the life of a catalyst containing ruthenium or ruthenium compounds for the oxidation of hydrogen chloride to reduce the sulfur load in the entry stream to below 2 ppm. For reducing the sulfur load, various oxides on which sulfur components are deposited by reaction are described in this application. A disadvantage of this process is that volatile chlorides of these elements may be entrained onto the catalyst or a peak contamination in the sulfur load may lead to a breakthrough of sulfur compounds.
Processes for the regeneration of catalysts containing ruthenium or ruthenium compounds and poisoned with sulfur in the form of sulfur compounds have already been described but have all kinds of disadvantages. GB 744 049 A discloses that catalysts containing ruthenium or ruthenium compounds and poisoned with sulfur in the form of sulfur compounds can be regenerated by scrubbing. Water, ethanol, acetic acid, cyclohexene, benzene and acetone are mentioned as examples of the wash liquid. However, scrubbing always entails the risk that a part of the active components will be discharged with the wash liquid. This can happen both through physicochemical processes (e.g. reaction+absorption, solubility) and through mechanical processes (e.g. abrasion). Furthermore, for scrubbing, the catalyst must as a rule be removed from the reactor used for the target reaction.
GB 1 278 119 A describes the regeneration of a catalyst containing ruthenium or ruthenium compounds and poisoned with sulfur in the form of sulfur compounds by a reducing treatment with an anhydrous hydrogen stream at 430 to 595° C., a pressure between 3 and 24 bar and some further oxidation and reduction steps. Such a combination of reducing conditions and high temperatures will lead to substantial reduction of ruthenium oxides (if present beforehand) to metallic ruthenium down to relatively deep layers. As a result of this treatment, the catalyst containing ruthenium or ruthenium compounds will be subjected to drastic changes which are probably undesired for some applications. Moreover, pressure-resistant reactors, pipelines and fittings must be available for this application, and for this reason the catalyst would as a rule have to be removed for this treatment.
Accordingly, no process which is simple to handle has as yet been developed, by means of which the regeneration of a catalyst containing ruthenium or ruthenium compounds and poisoned with sulfur in the form of sulfur compounds is possible under mild conditions. The processes known to date all entail the risk of a partial loss of ruthenium or an undesired change in the catalyst structure. Accordingly, gradual poisoning of a catalyst containing ruthenium or ruthenium compounds by sulfur in the form of sulfur compounds is still a limiting factor for the on-stream time in many processes. An abrupt, irreversible loss of activity due to an undesired peak in the sulfur load (e.g. owing to non-steady-state startup processes) represents the worst case scenario. Both factors entail an enormous economic risk since the recovery of the ruthenium from a spent catalyst is expensive and a partial ruthenium loss must be allowed for.